Crystallographic and Geometrical Dependence of Water Oxidation Activity in Co-Based Layered Hydroxides

Cobalt-based layered hydroxides (LHs) stand out as one of the best families of electroactive materials for the alkaline oxygen evolution reaction (OER). However, fundamental aspects such as the influence of the crystalline structure and its connection with the geometry of the catalytic sites remain poorly understood. Thus, to address this topic, we have conducted a thorough experimental and in silico study on the most important divalent Co-based LHs (i.e., α-LH, β-LH, and LDH), which allows us to understand the role of the layered structure and coordination environment of divalent Co atoms on the OER performance. The α-LH, containing both octahedral and tetrahedral sites, behaves as the best OER catalyst in comparison to the other phases, pointing out the role of the chemical nature of the crystalline structure. Indeed, density functional theory (DFT) calculations confirm the experimental results, which can be explained in terms of the more favorable reconstruction into an active Co(III)-based oxyhydroxide-like phase (dehydrogenation process) as well as the significantly lower calculated overpotential across the OER mechanism for the α-LH structure (exhibiting lower Egap). Furthermore, ex situ X-ray diffraction and absorption spectroscopy reveal the permanent transformation of the α-LH phase into a highly reactive oxyhydroxide-like stable structure under ambient conditions. Hence, our findings highlight the key role of tetrahedral sites on the electronic properties of the LH structure as well as their inherent reactivity toward OER catalysis, paving the way for the rational design of more efficient and low-maintenance electrocatalysts.

sealed with Kapton ® tape (50 µm in thickness) to prevent the oxidation of the sample. The optimum amount of material for the measurements was calculated by the program hephaestus which is part of the Demeter package. 6 The pre and post-OER samples were measured over carbon paper electrodes before and after the OER catalysis. A Si(111) double-crystal monochromator was used to obtain a monochromatic incident beam over the sample, and the intensities of the incident and transmitted X-rays were measured using two ionization chambers, respectively. XAS spectra were collected from 7590-8550 eV with a reduced step (0.3 eV) in the XANES region (7690 to 7750 eV) for Co K-edge. The incident photon energy was calibrated using the first inflection point of the Co K-edge (7709 eV) from reference foils of metallic Co. For each sample, three spectra were taken with exposure times of 4 min for each one to later be averaged. XANES data treatment was performed by subtracting the preedge background followed by normalisation by extrapolation of a quadratic polynomial fitted at the post-edge region of the spectrum using ATHENA AUTOBK background removal algorithm. 7 The quantitative analysis of the EXAFS results were performed by modelling and fitting the isolated EXAFS oscillations. The EXAFS oscillations (k) were extracted from the experimental data with standard procedures using the Athena program. The k 2 weighted (k) data, to enhance the oscillations at higher k, were Fourier transformed. The Fourier transformation was calculated using the Hanning filtering function. EXAFS modelling was carried out using the ARTEMIS software. 6 Theoretical scattering path amplitudes and phase shifts for all paths used in the fits were calculated using the FEFF9 code. 8 The k-range was set from 2.5 to 12.3 Å -1 . The passive reduction factor S0 2 values were restrained to 0.8 for Co, respectively. These values were obtained from the fitting standard foils of metallic Co and constraining the coordination numbers to those corresponding to each structure.

Electrochemical Characterization
KOH purification. To purify the electrolyte (KOH), Ni fibers (BEK-POR 2N118-0.25, Bekaert. 99.9% purity) were used as both working and counter electrodes for a prolonged electrolysis process lasting 1 day at high current densities. This approach was motivated by previous reports. 9,10 Electrode Preparation. For the electrode preparation, a dispersion composed of 2,5 mg of powder material, 1 mL of water and ethanol (1:1) and 7 µL of Nafion (5 %) was sonicated in order to obtain a well-dispersed suspension. Then, 5.7 µL was drop-casted in a previously polished (sequentially with 1.0, 0.3 and 0.05 µm alumina powder) 3 mm glassy carbon electrode. Afterwards, the solvent was let evaporated at room temperature. The electrode mass loading achieved was around 0.20 mg•cm -2 . On the other hand, modified carbon paper electrodes were prepared by spray coating the previous dispersion (using an airbrush from Harder Evolution) on carbon papers with a geometrical area of 2 x 1 cm 2 . The electrode mass loading achieved was around 0.50 mg•cm -2 . Electrochemical Measurements.
Electrochemical tests were performed in a three-electrode cell equipped with glassy carbon acting as the working electrode and a platinum wire as the counter electrode. As the reference electrode, a silver-silver chloride (Ag/AgCl (3 M KCl)) was used. All potentials were converted referring to the oxygen evolution reaction overpotential. The measurements were performed on an Autolab PGSTAT 128N potentiostat/galvanostat. Linear sweep voltammetry (LSV) measurements were carried out at 5 mV•s −1 in a previously N2 purged 1 M KOH aqueous solution. Prior to this, 30 cyclic voltammetry measurements were performed at 50 mV•s -1 to activate the material. Similar conditions were used to carry out the measurements on modified carbon paper electrodes. However, in this case, another carbon paper with higher surface area (3 x 3 cm 2 ) was used as the counter electrode.
Electrochemical surface area was acquired by measuring the current associated with doublelayer capacitance from the scan rate dependence of CVs. The potential range used for the CVs was from -0.25 to -0.05 V versus Ag/AgCl (3 M KCl). The scan rates were 300, 250, 200, 150, 100 and 50 mV·s -1 . The double layer capacitance was estimated by plotting the (ja-jc) (anodic versus cathodic currents) at -0.15 V versus Ag/AgCl (3 M KCl) against the scan rate. The ECSAs were measured on the working electrodes after performing an activation process consisting of 10 CVs at 50 mV•s -1 around their redox processes.
The turnover frequency (TOF) values were calculated from the following equation: where j is the current density at a given overpotential, A is the surface area of the working electrode, F is the Faraday constant, and n is the total number of moles of the material.
Electrochemical impedance spectroscopy (EIS) measurements were carried out using a Gamry 1000E potentiostat/galvanostat controlled by Gamry software by applying an AC amplitude of 10 mV in the frequency range of 100-105 Hz at an overpotential of 0.40 V. EIS data were analysed and fitted by means of Gamry Echem Analyst v. 7.07 software.
Electrochemical stability tests were done using an Autolab PGSTAT 128N potentiostat/galvanostat. At first, samples were subjected three times to 30 activation cycles, an LSV and an OFF time of 10 min.

DFT+U calculations
All calculations were performed in periodic boundary conditions employing density functional theory (DFT) as implemented in the Quantum Espresso code, 11 which is based on the pseudopotential approximation to represent the ion-electron interactions, and plane waves basis sets to expand the Kohn-Sham orbitals. Ultrasoft-type pseudopotentials were adopted, in combination with the PBE formalism to compute the exchange-correlation term. 12 The magnetic states are described through the Kohn-Sham Hamiltonian in the framework of spinpolarized calculations, plus a Hubbard term. On the basis of our previous reports the incidence of the Hubbard parameter in the DFT+U calculations on the magnetic coupling and other properties was fixed to 4.5 eV for Co atoms.
In all cases, spin-orbit contributions were considered. 13 An energy threshold of 10-8 au was used for self-consistency, while for geometry optimization the convergence criteria were 10-6 au for the energy and of 10-3 au for the forces per atom. To improve the numerical convergence a first-order Methfessel-Paxton spreading was implemented. van der Waals interactions were considered by including the semiempirical correction DFT-D originally introduced by Grimme 14 and implemented in a plane-wave framework by Barone and coworkers. 15 The simulations were carried out on supercells with specific ordering of the metal polyhedra within the layers. Brillouin zone sampling was performed on these supercells with a Monkhorst-Pack grid, checking for convergence with respect to the number of k-points. A 6x6x1 k-point grid was used in both cases. The atomic structures reported in this work were visualised using XCrysDen. 16 OER mechanism. We employed the computational hydrogen electrode (CHE) method developed by Nørskov. 17 It assumes the chemical potential of a proton-electron pair equals that of gas-phase H2 in standard condition, G(H++e) =G(H2). To each step of the OER mechanism we estimated the Gibbs free energy change, ΔG=ΔE+ZPE−TΔS at a standard condition where ΔE is the DFT+U total energy of the system substrate + adsorbate, ZPE is the zero-point energy18 and the last term correspond to the entropic correction calculated at a temperature of 298.15 K. In this framework, the global reaction 4OH -→ 2H2O + O2 + 4einvolves a standard Gibbs free energy change of 4.92 eV at room temperature. Harmonic approximation was employed to treat the allowed vibrations and the molecular partition equation of statistical mechanics under standard conditions was used to obtain the entropic corrections. The potential bias effect was included on all states involving an electron in the electrode, by ΔGU=-eU, where U is the electrode potential.
The reaction free energy is then calculated as ΔG(U, pH2=1bar, T)=ΔG + ΔGU The reference energy for the gaseous O2 was corrected to yield the calculated total energy sum of the OER pathway as 1.   Figure S2. UV-Vis spectra of the Co-based LHs family highlighting the position of the peaks (see Table S3 for further details). Table S3. UV-Vis signals of the Co-based LHs family as presented in Figure S2.

Sample
β-LH 5.9 2.09 0.008 6.       Figure S7. Reaction standard free energy diagrams for the OER process, at zero potential (U = 0, filled lines) and equilibrium potential for oxygen evolution (U = 1.23, dashed lines), for each OH type on the LHs structures. These results show that equilibrium potential (1.23) is not enough for all steps to be thermodynamically favourable.
ɑ-LH (post OER) 5   Figure S11. Electrochemical Impedance spectroscopy characterisation before and after the long-term stability tests (see Figure S9) of the different Co-based LH samples deposited on carbon paper and recorded at an overpotential of 0.45 V. β-LH (A), LDH (B) and ɑ-LH (C).